My 4-year-old DD went to a couple of ultrasounds with me (I've had 5). During the NT scan, the paperwork said that kids 4 and under needed another adult to supervise them--but I didn't know that until I lugged her along to the scan by myself. Nice to them to let me know ahead of time, right? Thankfully DD is pretty calm and behaved herself, so the tech let it go. For the ultrasounds that were done in my OB's office, they never said a word about her not being allowed in there. She was actually pretty fascinated with what she saw but eventually just wanted to entertain the tech and talk her ear off. The policy about kids must vary from office to office.